Re: [PATCH] Add a page cache-backed balloon device driver.

On Tue, 26 Jun 2012 16:21:58 -0700, Frank Swiderski [off-list ref] wrote:

On Tue, Jun 26, 2012 at 2:47 PM, Michael S. Tsirkin [off-list ref] wrote:

quoted

Let's assume it's a feature bit: how would you
formulate what the feature does *from host point of view*?

In this implementation, the host doesn't keep track of pages in the
balloon, as there is no explicit deflate path.  The host device for
this implementation should merely, for example, MADV_DONTNEED on the
pages sent in an inflate.  Thus, the inflate becomes a notification
that the guest doesn't need those pages mapped in, but that they
should be available if the guest touches them.  In that sense, it's
not a rigid shrink of guest memory.  I'm not sure what I'd call the
feature bit though.

Was that the question you were asking, or did I misread?

Hmm, the spec is unfortunately vague: !VIRTIO_BALLOON_F_MUST_TELL_HOST
implies you should tell the host (eventually).  I don't know if any
implementations actually care though.

We could add a VIRTIO_BALLOON_F_NEVER_TELL_DEFLATE which would mean the
deflate vq need not be used at all.

Is it altogether impossible to know when a page is reused in your
implementation?  If we could do that, we could replace our balloon with
this one.

(My deep ignorance of vm issues is hurting us here, sorry.)

Cheers,
Rusty.